Would it be possible to take a flat bed heavy metal hay trailer and convert it into a working horse trailer? Use the flat bed as the base and cut and weld up walls/ ramps/ dividers/ doors/ windows/ etc.? Just whatever you wanted! You could totally customize one with whatever you wanted to add into it. Everything from living quarters to easy access doors, suicide doors, tack compartment with saddle racks and bridle pegs. You could add tie rings where ever you found necessary. You could include slants the horses had to back out of or a full stall size spot to turn them around or to put them in for over night trips. You could even choose between reg. Hitch or goose neck based on what flat bed you start with.
It was just an idea I had. And I figured anyone with Good cutting and welding skills could handle it. So I figured I would ask and see what others thought of the idea.